A magnetically shielded room, commonly referred to as an "MSR", is used to magnetically and electromagnetically isolate sensitive instruments and equipment such as highly sensitive biomagnetic detectors, measuring instruments and super conductive quantum interference devices, commonly referred to as "SQUIDs." Some of these sensitive instruments are used for biomagnetic research and tests to measure minute magnetic fields. To obtain proper readings and results from the sensitive instruments, the environment for conducting these tests must be extremely magnetically quiet. The MSR provides a magnetically and electromagnetically isolated environment for using this equipment by attenuating stray magnetic fields and other magnetic disturbances created or generated by the earth's magnetic field, power lines, current carrying conductors and moving metal masses such as trains, automobiles, elevators and other large metal objects in the vicinity of the MSR. Except for the power lines which create 50/60 hertz frequencies, the above mentioned influences create low or very low frequencies which often approach DC or zero. The earth's magnetic field is in fact a DC field. Since magnetic and electromagnetic shielding of DC to high frequencies is somewhat difficult to obtain, the ideal MSR includes multiple shielding layers or shields of very high magnetic permeability and highly electrically conductive materials capable of attenuating these various frequencies.
The MSR must also provide a stable and homogeneous environment for the small, if any, residual field in the MSR near the sensitive instruments. To achieve this stable and homogeneous environment, the MSR is preferably a six sided cube having a floor, a ceiling, four walls and a door in one of the walls. In the ideal MSR, the floor, ceiling, walls and door have multiple magnetic and electromagnetic shielding layers or shields to obtain high attenuation characteristics and to produce the desired stable and homogeneous environment in the interior chamber of the MSR.
An MSR may lose its attenuation, stability and homogenous characteristics due to vibration from the earth or parent building, the pressure or weight of the sensitive instruments on the weight bearing floor in the MSR and the normal use of the MSR including people walking on the weight bearing floor in the room. Such vibration, pressure, weight and use of the MSR can cause the shielding layers or shields to lose part of their attenuation properties which alter the readings from the sensitive instrument in the MSR, and ultimately the results of the biomagnetic or other research or tests. The ideal MSR room thereby compensates for the weight of the sensitive instrument by separating the weight bearing floor from the shielding layers or shields, and dampens the vibration caused by use of the room as well as from the earth and the parent building.
The use of multiple shielding layers or shields in an MSR creates additional problems, such as electric conductivity or magnetic coupling between the shielding layers or shields. Such electric conductivity may be eliminated by a dielectric separation between the shields, and such magnetic coupling may be substantially reduced by the appropriate separation or spacing of the independent shielding layers or shields with non-magnetically permeable material. In an ideal MSR, at one or more levels, at least one electrically conductive shielding layer or shield is thereby magnetically decoupled and electrically isolated from at least one magnetically permeable shielding layer or shield.